1. Field of the Invention
The present invention relates to power line communications, and more particularly, to a signal coupling apparatus for power line communications using a three-phase four-wire power line.
2. Description of the Related Art
In general, power line communications are used to transmit communication signals, for example, digital data signals whose frequency is in the range from several hundreds of kHz to several dozens of MHz, over a power line or to a distribution line. In the power line communication technology, a communication signal transmitted through a power line is extracted by a high pass filter.
In particular, the power line communication technology is convenient and economical because it provides networking such as Internet connection through an existing power line without a separate communication line.
Meanwhile, since power lines are designed to supply power of 60 Hz, they have different electrical characteristics in a frequency band of 2 through 30 MHz, which is a frequency band for high data rate communications. Accordingly, when such power lines are used for communications, the power lines have many disadvantages, such as attenuation, noise, delay, etc.
In order to overcome such disadvantages, a signal coupling apparatus has been conventionally utilized between a plug connected to a power line and a communication plug connected to a terminal. A conventional signal coupling apparatus carries signals onto a power line or extracts signals from a power line using band-pass filtering or impedance matching.
However, since the conventional technique requires a separate impedance matching apparatus as well as the signal coupling apparatus for power line communications, there are difficulties in installation and management. In addition, a signal coupling apparatus is required to be designed in consideration of a three-phase four-wire distribution line.
Accordingly, studies on a signal coupling apparatus which can have simple structure, match an impedance of each of three-phase power lines, and control a transmission mode, without using a separate impedance matching apparatus, are underway.